Opto-isolators, which are commonly referred to as optically toggled switches, are useful devices because of the complete electrical isolation that they provide between an electrical input circuit and an electrical output circuit. An opto-isolator comprises a light source in an electrical input circuit and a photodetector in an electrical output circuit. The light source and photodetector are optically coupled to each other and the electrical output circuit changes its current conduction characteristics when the amount of light received by the photodetector changes.
Recently, optically toggled switches using a photodiode array which, upon illumination, produces a voltage that controls the current conduction state of a field effect transistor (FET) have been developed. One such switch uses a single photodiode array to control the current conduction state of a single normally ON JFET. For example, U.S. Patent application Ser. No. 154,856, filed on May 30, 1980 and assigned to the assignee of this invention, describes such a switch. Another such switch uses a photodiode array to control the current conduction state of a normally OFF vertical metal oxide semiconductor (VMOS) FET to form a linear unilateral switch. The structure and characteristics of VMOS FETs are described in IEEE Transactions on Electron Devices, pp. 1222-1228 and pp. 1229-1234, ED-25, October, 1978. If a linear bilateral switch is desired, the addition of a second VMOX FET, with the FET source electrodes connected to each other, yields such a switch.
While this switch is adequate for many applications, it has at least one drawback. For example, the two FETs are not readily integrable on a single chip because the source electrode of the VMOS FET is located on the top surface of the chip and consequently, connections between the two sources cannot be made on the chip itself. As a result, two VMOS FET chips are required.
An optically toggled normally OFF switch such as that just described can be usefully employed in many applications both by itself and with other types of switches. For example, many telephony applications require voltages of different magnitudes and it is often undesirable to have the larger voltage applied to the circuit that carries the smaller voltage. One such telephony application arises because the ringing voltage is commonly 105 volts and the tone generator is commonly 48 volts. It is desirable to break the circuit between the station apparatus and the central office tone generator before the ringing voltage is applied to the station apparatus so that the ringing voltage never reaches the tone generator circuit. Such operation can be obtained with what is termed a break-before-make switch. For the application being discussed, the break-before-make switch has, in the absence of a ringing voltage, a normally ON switch which connects the tone generator and station apparatus. It also has a normally OFF switch between the ringing voltage and the station apparatus. When the application of the ringing voltage is desired, the normally ON switch turns OFF before the ringing voltage is applied to the station apparatus. This prevents the ringing voltage from being applied to the tone generator circuit or other central office equipment which may be designed to handle only 48 volts. When the ringing voltage is no longer desired, the normally OFF switch connecting the ringing voltage to the station apparatus opens before the normally ON switch closes and the tone generator voltage is again applied to the station apparatus.